The most familiar air moving mechanism is the simple axial fan, which is the stationary equivalent of an airplane propeller. Whether used for residential cooling or automotive radiator cooling, it simply pulls air axially straight through it. Less familiar is the so called centrifugal blower, which finds common usage in vehicle HVAC systems. A centrifugal blower has a generally cylindrical impeller or fan rotating in one direction that pulls air in along its central axis as it rotates, but then forces it radially outwardly, turning it ninety degrees, in effect. A scroll shaped blower housing surrounding the impeller collects and confines the expelled air and sends it through a tangential outlet to the rest of the HVAC system.
The basic cylindrical impeller includes a central hub, often dome shaped, through which a motor drive shaft is attached, and a flat, annular outer rim. Extending upwardly from the hub rim are an evenly spaced series of identical blades, which are parallel to the central axis.
Plastic centrifugal fans for HVAC applications have traditionally had outer rims that are either external to the blade (external rims) or above part of the blade (split louver rims). Both external rims and split louver rims have high stress areas between the blades as the fan is spun to its operating speed, known as hoop stress. Hoop stress requires the use of higher strength materials, which generally have higher mass, and are more costly.
In attempting to minimize hoop stress, prior art designs have used external rim designs with a long axial length or a split louver design with a parabolic shaped rim of constant thickness. Both of these designs, add axial length and reduce noise, however, they do not add strength to the overall design and require strong, plastic materials with fillers that increase mass, cost, and variation in as-molded balance.
The present invention is aimed at one or more of the problems identified above.